Enable Use of Strict Mode Checking

An XCCDF Rule

Description

SSHs StrictModes option checks file and ownership permissions in the user's home directory .ssh folder before accepting login. If world- writable permissions are found, logon is rejected.
The default SSH configuration has StrictModes enabled. The appropriate configuration is used if no value is set for StrictModes.
To explicitly enable StrictModes in SSH, add or correct the following line in /etc/ssh/sshd_config:

StrictModes yes

Rationale

If other users have access to modify user-specific SSH configuration files, they may be able to log into the system as another user.

ID: xccdf_org.ssgproject.content_rule_sshd_enable_strictmodes
Severity: Medium
References: 12 13 14 15 16 18 3 5

APO01.06 DSS05.04 DSS05.07 DSS06.02

3.1.12

CCI-000366

164.308(a)(4)(i) 164.308(b)(1) 164.308(b)(3) 164.310(b) 164.312(e)(1) 164.312(e)(2)(ii)

4.3.3.7.3

SR 2.1 SR 5.2

A.10.1.1 A.11.1.4 A.11.1.5 A.11.2.1 A.13.1.1 A.13.1.3 A.13.2.1 A.13.2.3 A.13.2.4 A.14.1.2 A.14.1.3 A.6.1.2 A.7.1.1 A.7.1.2 A.7.3.1 A.8.2.2 A.8.2.3 A.9.1.1 A.9.1.2 A.9.2.3 A.9.4.1 A.9.4.4 A.9.4.5

CIP-003-8 R5.1.1 CIP-003-8 R5.3 CIP-004-6 R2.3 CIP-007-3 R2.1 CIP-007-3 R2.2 CIP-007-3 R2.3 CIP-007-3 R5.1 CIP-007-3 R5.1.1 CIP-007-3 R5.1.2

AC-17(a) AC-6 CM-6(a)

PR.AC-4 PR.DS-5

SRG-OS-000480-GPOS-00227
Updated: about 1 year ago

- name: Find sshd_config included files
  shell: |-
    included_files=$(grep -oP "^\s*(?i)include.*" /etc/ssh/sshd_config | sed -e 's/\s*Include\s*//i' | sed -e 's|^[^/]|/etc/ssh/&|')
    [[ -n $included_files ]] && ls $included_files || true
  register: sshd_config_included_files
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]  tags:
  - NIST-800-171-3.1.12
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-6
  - NIST-800-53-CM-6(a)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_strictmodes

- name: Comment conf from included files
  replace:
    path: '{{ item }}'
    regexp: ^(\s*StrictModes.*)$
    replace: '# \1'
  loop: '{{ sshd_config_included_files.stdout_lines }}'
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-171-3.1.12
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-6
  - NIST-800-53-CM-6(a)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_strictmodes

- name: Enable Use of Strict Mode Checking
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*StrictModes\s+
      state: absent
    check_mode: true
    changed_when: false
    register: dupes

  - name: Deduplicate values from /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*StrictModes\s+
      state: absent
    when: dupes.found is defined and dupes.found > 1

  - name: Insert correct line to /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*StrictModes\s+
      line: StrictModes yes
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-171-3.1.12
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-6
  - NIST-800-53-CM-6(a)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_strictmodes

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

# Find the include keyword, extract from the line the glob expression representing included files.
# And if it is a relative path prepend '/etc/ssh/'
included_files=$(grep -oP "^\s*(?i)include.*" /etc/ssh/sshd_config | sed -e 's/\s*include\s*//I' | sed -e 's|^[^/]|/etc/ssh/&|')for included_file in ${included_files} ; do
    
    LC_ALL=C sed -i "/^\s*StrictModes/Id" "$included_file"
done

if [ -e "/etc/ssh/sshd_config" ] ; then
    
    LC_ALL=C sed -i "/^\s*StrictModes\s\+/Id" "/etc/ssh/sshd_config"
else
    touch "/etc/ssh/sshd_config"
fi
# make sure file has newline at the end
sed -i -e '$a\' "/etc/ssh/sshd_config"

cp "/etc/ssh/sshd_config" "/etc/ssh/sshd_config.bak"
# Insert at the beginning of the file
printf '%s\n' "StrictModes yes" > "/etc/ssh/sshd_config"
cat "/etc/ssh/sshd_config.bak" >> "/etc/ssh/sshd_config"
# Clean up after ourselves.
rm "/etc/ssh/sshd_config.bak"

else
    >&2 echo 'Remediation is not applicable, nothing was done'
fi

Enable Use of Strict Mode Checking

Description

Rationale

Remediation - Ansible

Remediation - Shell Script